1. Field of the Invention
The invention relates to a method of allocating a right to transmit to different stations which form part of a decentralized communication network and each of which is connected to a data traffic channel, the right to transmit being allocated to the various stations by means of a token in an order to be determined, at least one priority level for the allocation frequency of the token being assigned to each station, the stations having the same priority level forming part of the same group and the network comprising at least two different groups.
2. Description of the Prior Art
A method of this kind is described, for example, in an article by M. Stieglitz, entitled "An LSI token Controller", published in IEEE Digest of Papers, Compcon Spring 1982, Twenty-Fourth IEEE Computer Society International Conference, Feb. 22-25, 1982, San Francisco, Calif., pages 115-120. The various stations form part of decentralized communication network. Contrary to a centralized communication network, such a decentralized network does not comprise a pilot station which manages the protocol for the various stations for access to the channel. The token circulates on the channel and grants the right to transmit to the various stations in a predetermined order. This predetermined order is not necessarily the order in which the stations are physically connected to the network, but rather a logic order.
There are several general transmission modes for the token. The transmission of the token may be performed in a first mode which is referred to as the random mode, for example, as in the method known as CSMACD (Carrier Sense Multiple Access with Collision Detection); according to this method, each station may claim the token at any instant.
The transmission of the token may also be performed in a second mode, referred to as the designated mode; this mode is either implicit as in the method described in U.S. Pat. No. 4,495,493, explicit by utilizing a particular transmission frame of the token which contains the functional or physical address of the station whereto the token will be allocated.
The method in accordance with the present invention concerns the transmission of the token according to said explicit designated mode. Certain stations have a priority which is higher than that of other stations; this means that during an allocation cycle of the token during which all stations receive the right to transmit, such stations of higher priority will receive this right more often than the other stations.
According to the method disclosed in said article, the use of the token allocated to the stations of lower priority is deferred in order to allocate the token more often to the stations of higher priority. It is a drawback of this method that when there are stations having a very high priority level, the stations which have a low priority level can only pass on the token without using it, thus causing network overloading, so that the allocation of the token to the stations of higher priorities is delayed.
In the known explicit designated modes all stations are connected to the same logic loop; each station knows its successor at any instant, even though the successor of a station may change in time due to the starting and stopping of the various stations. In any case, according to the known methods the stations of the network are connected only to a single address; moreover, the token circulates only cyclically on the channel so that a station is obliged to wait for its predecessor to pass on the token in order to enable transmission.
These characteristics of the known methods are not without drawbacks, because there are often given stations which have to transmit with well defined time intervals. The method in accordance with the invention has for its object to mitigate such drawbacks.